(1) Field of the Invention
The present invention relates to a coated paper sheet. More preferably, the present invention relates to a coated paper sheet having a high gloss, a high Clark stiffness determined in a cross direction (CD) of the sheet and being free from generation of blistering on the sheet when subjected to offset printing or full color printing by, for example, an electrophotographic printing system, and thus exhibiting an excellent passing property through the printing system and a superior capability of being printed with printed images having high quality, namely high clarity and sharpness.
(2) Description of the Related Art
Currently, due to a strong demand of full color printing, high speed printing and high image quality printing, a large amount of coated paper sheets are consumed in the printing and publishing business. Particularly, in the field of on-demand-printing, there is a significant trend of preparing various publications, which have been prepared by the conventional printing method, by a color copying machine or a color printer which can relatively easily respond to a small order. Also, application of coated paper sheets having a high white gloss, in place of conventional paper sheets for plain paper copiers (PPC) and for printers, to electrophotographic copiers and printers, are now increasing.
To respond to the above-mentioned trend, the coated paper sheets for the on-demand publishing must provide sufficient aptitude both for off-set press and for electrophotographic copying machine and printers. For the off-set press, the recording sheets must have a high surface strength (toughness) and a high blister resistance. Also, for electrophotographic full color copying machine and printers, the recording sheet must have a high stiffness and a high blister resistance. Further, to prepare high grade printing, the recording paper sheet is required to have an improved stiffness, because conventional coated paper sheets having a high white gloss are unsatisfactory for high grade printing.
Generally, the coated paper sheet having a high white gloss are produced by coating a substrate paper sheet with a coating containing a white pigment having an average particle size of 2 μm or less, in a dry solid amount of 10 g/m2 or more per surface of the sheet, and smoothing the coating layer surface by calendering. In this case, it is known that the coated paper sheets having a high white gloss is compressed by the calendering and thus the stiffness of the coated paper sheets decreases.
Further, in view of various social requirements due to developments in movements for conservation of natural resources and protection of natural environment and other requirements to decrease a load of consumers on transportation of the prints, and to scale-down of the space for storing the prints in home and library, it is expected that the mass of the recording paper sheets will be further decreased. However, generally speaking, the decrease in mass of the paper sheets causes the thickness and stiffness of the paper sheets to be decreased and thus a problem, that the printed products appear to be a low grade and a difficulty in turning over the leaves of printed products and in reading the printed products increases, occurs.
In the offset sheet press or the electrophotographic copying machine or printer, the stiffness of the paper sheets greatly influences on the passing property of the paper sheets through the printer, and thus it is severly controlled as an important quality item. Especially, in the electrophotographic copying machine and printer, when a paper sheets having a low stiffness are employed, the paper sheets may sometimes be fed irregularly into the printer, are not smoothly passed through the printer and may block the delivery from the printer.
As one way of enhancing the stiffness of the coated paper sheet, the substrate paper sheet is produced from a mechanical pulp which contributes to enhancing the stiffness of the substrate paper sheet and thus of the resultant coated paper sheet. However, the use of the mechanical pulp causes the resultant coated paper sheet to exhibit a low smoothness and a reduced whiteness and it appears to be a low grade. Also, when a chemical pulp produced mainly from soft woods, for example, NBKP is used in a major proportion to produce the paper sheet substrate, the resultant paper sheet exhibits a high stiffness. However, the air permeance of the resultant paper sheet is undesirably increased.
It has been attempted to increase the stiffness of the paper sheet by make the paper sheet bulky. The bulkiness of the paper sheet can be increased by increasing the freeness of the pulp, by reducing the pressure of a wet press or by reducing the pressure of calendering. However, while the increase in bulkiness contributes to enhancing the stiffness of the resultant paper sheet, the resultant paper sheet has a porous inside structure and a low smoothness. When the porous paper sheet is coated with a pigment-containing coating liquid, a great portion of the coating liquid permeates inside the porous paper sheet, and thus the coverage of the coating liquid over the surface of the porous paper sheet decreases. Therefore, after drying, the resultant coated paper sheet is unsatisfactory due to a low gloss and a low uniformity of the gloss. The coated paper sheet having an insufficient gloss must be smoothed by calendering under increased pressure. Thus, the resultant calendered paper sheet exhibits an increased compressive modulus and a greatly decreased stiffness.
Generally, it is known that when an attempt is made to improve the quality of printed images by enhancing the gloss of the paper sheet, the resultant paper sheet has a low stiffness and thus exhibits a poor passing property through the printer. To solve this problem, Japanese Unexamined Patent Publication No. 5-341,553 discloses an attempt to solve the problem of the poor passing property of the paper sheet by controlling a basis mass of the paper mass in the range of from 75 to 95 g/m2.
Further, in the production of a coated paper sheet having a high white gloss, an application of a calendering treatment causes the paper sheet substrate and the coating layer of the calendered coated paper sheet to respectively exhibit increased densities and the resultant coated paper sheet to exhibit a significantly decreased air permeance. In the case where the coated paper sheet having the low air permeance is subjected to a printing procedure using an offset press or an electrophotographic copying machine or printer, a problem such that when the printed ink images are dried or the toner images are heat-fixed, blisters are generated in the coated paper sheet, occurs. The bisters generated in the coated paper sheet are classified into macro-blisters and micro blisters. The macro-blisters are generated by deforming the substrate and the coating layer due to expansion of water contained in the substrate. The macro-blisters are formed in the image-printed portions of the coated paper sheet. The micro-blisters are fine bisters generated in the image-printed portions of the coated paper sheet due to expansion of water vapor generated between the coating layer and the ink or toner images printed on the coating layer. The micro-blisters cause a decrease in gloss of the printed coated paper sheet.
As a general countermeasure to macro-bister generation, there is a method of preventing the breakage of the paper sheet substrate by enhancing the internal bond strength of the substrate. As a method of enhancing the internal bond strength of the substrate, an employment of pulp fibers prepared under an intensified beating condition and having a high bonding strength to each other to form the paper sheet substrate, a coating or impregnating of the paper sheet substrate with a resin and a mixing of a paper strength-enhancing agent into the paper sheet for the substrate, are effectively utilized. However, the use of the pulp fibers produced under a intensified beating condition and the impregnation or coating with a resin contribute to enhancing the internal bond strength of the paper sheet substrate, but cause the air permeance of the paper sheet substrate to decrease. Therefore, these measures can control only the generation of macro-blisters but not of micro-blisters. As a measure of mixing the paper strength-enhancing agent into the paper-forming pulp slurry, Japanese Unexamined Patent Publication No. 3-227,491 discloses mixing a polyacrylamide-compound-containing paper strength-enhancing agent into a paper-forming pulp slurry. In this case, however, when the paper strength-enhancing agent is mixed in too large an amount, flocks are generated in the pulp slurry. Therefore, in order to maintain the process conditions and the uniformity of the resultant paper sheet at high level, the paper strength-enhancing agent must be employed in a limited amount.
Thus, it is difficult to obtain a satisfactory effect on the prevention of the generation of blisters by the use of a paper strength-enhancing agent.
To prevent the generation of the micro-blisters, an increase in the air permeance of the coating layer is considered effective. However, in the coating layer, the higher the air permeance, the lower the white gloss. Accordingly, the conventional coated paper sheet having a high white gloss exhibits an insufficient resistance to micro-blister generation.
As Japanese Unexamined Patent Publication No. 11-174,713 discloses, it is known that, when coated paper sheets having a high white gloss are printed by an offset sheet press or electrophotographic copying machine or printer, particularly in high humid circumstances and, for example, in a rainy season, a closed package of the coated paper sheets is opened and the coated paper sheets are subjected to printing, immediately after the start of printing, misfeeding of a plurality of superposed paper sheets, or blocking of the paper sheet delivery, often occur. The reason of the above-mentioned problem is assumed to be that the coated paper sheets with a high white gloss have a high smoothness and thus are easily adhered to one another, and the surface tension and hydrogen bond of water absorbed in the coating layers of the coated paper sheets cause the coated paper sheets to be adhered one another, and the increased static friction coefficient between the coated paper sheets adjacent to each other, which friction prevents the smooth passing of the coated paper sheets through the printer, increases.
As stated above, when subjected to an offset printing procedure or an electrophotographic full color copying machine, the coated paper sheet having a high 75 degrees specular gloss of 70% or more and a basis mass of, for example, 70 to 90 g/m2 and further exhibiting both a high resistance to blistering and an excellent passing property through the printer or copying machine during the printing procedure have not yet been provided, in practice.